Capacitors, such as electrochemical capacitors, generally include outer substrates that are electrically conductive. Inner surfaces of the substrates are coated with electroactive material to form electrodes between which an electrolyte is sandwiched to form a capacitor. A capacitor including a single electrolyte layer between electroactive layers is often referred to as a single-cell capacitor.
Capacitors can also include multiple cells that are connected in series to obtain a higher voltage device. Each cell in the series stack is constructed from an electrolyte sandwiched between electroactive material. The multiple cells are separated by one another by electrically conductive substrates. However, such multiple-cell capacitors can be damaged if operated at or close to the rated capacitor voltage for times greater than the time constant of the cell with the highest leakage current, i.e., the most lossy cell, in a capacitor stack. The voltage over the lossy cell decreases and, as a result, the voltages over the other cells increase to maintain the applied (supply) voltage. This higher voltage can damage the cells and cause instabilities arising due to variations in the leakage current and/or leakage resistance between cells in the multi-cell capacitor.
Thus, what is needed is a way to balance cells within a multiple cell capacitor to prevent occurrences of voltage variation and device instability.